This proposal, from five investigators in three colleges at Oregon State University, would allow purchase of an automated optical biosensor. Two competing biosensor instruments are on the market, each capable of performing quantitative analysis, in real time, of protein-protein and protein-nucleic acid associations and dissociations. We have chosen the BIAcore 2000, a fully automated instrument that measures an optical property called surface plasmon resonance. In practical terms, this means that one can follow binding of a protein to a previously immobilized protein or nucleic acid. Once an association reaction is complete, one then removes the protein in solution and follows dissociation of the complex, with both association and dissociation followed in real time. Thus, one can determine binding on and off rates, dissociation constants, and binding stoichiometries. The measurements use small amounts of material, and the process is automated, for repetitive unmonitored analyses. Simple modifications permit analysis of assembly of multiprotein complexes and the effects of small molecules on binding equilibria. Planned applications include research on protein-protein interactions in DNA precursor biosynthesis (Mathews), analyses of protein-nucleic acid interactions in conjunction with mass spectrometry (Barofsky), RNA-protein interactions in plant virus RNA replication (Dreher), ligand-induced conformational changes in retinoid receptors (Leid), and affinity ligands for cellulolytic enzymes (Penner).